Almost any reasonably-sized supermarket stock camping gaz 907 904 and the smaller cartridges. They used to be cheaper than the UK, but who knows with the exchange rate what it is. There was a problem a couple of years ago when there was a complete dearth of refils in the LM area - all supermarches had sold out and I eventually got one in a small 'outdoor' shop in Arnage. If you are taking an empty bottle back, go first to the accueil in the supermarket where they will take the empty and give you a ticket that you present to the checkout to save you having to pay the deposit (again).
You will not get refills for any other bottle that is available in the UK (Calor, Flogas etc) as, as has effectively already been pointed out, the regulators are unique to each manufacturer. In an emergency, there is a plentiful supply of the larger bottles from supermarkets, garages etc, but you will have to pay a deposit on the bottle plus buy a regulator.
Summary: Stick with Camping Gaz or take enough with you.
Del